Electromagnetic interference (EMI) filters are required in various situations, such as in relation to power supply of electronic circuits like electronic circuits on circuit boards or circuit blades, for instance for powering telecommunication or data communication circuits.
Such boards or blades are then typically powered from a system voltage, for instance 48V DC. Because of this they may also require EMI filtering on their power inputs in order to pass conducted emission requirements.
The conducted emission may for instance be measured according to CISPR 22 or some similar standard. The requirement on a board or blade can be according to Class A for enterprise products or Class B for consumer products or any other level determined by a system designer. The noise that needs to be filtered then typically stems from one or more DC/DC converters.
An EMI filter can either be realized as a discrete design or in the form of a module containing the filter.
However, there is a problem in that the power consumption of the boards or blades increase and with this the size of the EMI-filter. Board space and building height is, however, limited. It would therefore be advantageous if the power level could increase without requiring more board space for the EMI-filter.
Furthermore, in these filters there is used a pair of magnetically coupled coils operating as common mode chokes. The traditional way of realizing these coils is through toroid chokes.
However, commonly available toroid chokes grow big in comparison with other circuits on the board or blade when currents exceed 10A.
In order to reduce the size of EMI-filters, investigations have been made directed towards making the windings planar.
One document in this field is US 2008/0309431, which is directed towards an EMI filter comprising a pair of opposed coreless spiral planar windings and a planar capacitor in order to integrate common mode and differential mode filters into integrated planar structures.
Another document describing planar windings in EMI filters is “Planar Electromagnetic Integration Technologies for Integrated EMI Filters”, by Rengang Chen, J. D. van Wyk, S. Wang and W. G. Odendaal, 38th IAS Annual Meeting. Conference Record of the Industry Applications Conference, 2003, October 2003, page(s): 1582-1588.Vol 3.
However, there is still a need for further measures that allow further improvements and size reductions.